1. Field of the Invention
The present invention relates to a silica sol composition, to a membrane electrode assembly with a proton-exchange membrane, and to a fuel cell comprising the assembly.
2. Description of the Background
These days it is expected that solid polymer fuel cells will be put into practical use for, for example, power sources for household use and power sources to be mounted on vehicles as clean power-generating devices that are ecological to the global environment. The main stream of such solid polymer fuel cells is toward those that require hydrogen and oxygen as the fuel thereof. Recently, a direct methanol fuel cell (DMFC) has been proposed, in which methanol is used in place of hydrogen for fuel. This is expected to give high-capacity batteries for mobile devices that are substitutable for lithium secondary batteries, and is now much studied in the art.
The important functions of the electrolytic membrane (proton-exchange membrane) for solid polymer fuel cells are to physically insulate the fuel (e.g., hydrogen, aqueous methanol solution) fed to the anode, catalyst electrode from the oxidizing gas (e.g., oxygen) fed to the cathode, to electrically insulate the anode from the cathode, and to transmit the proton having been formed on the anode to the cathode. To fulfill these functions, the electrolytic membrane must have some mechanical strength and proton conductivity.
In the electrolytic membrane for solid polymer fuel cells, generally used is a sulfonic acid group-having perfluorocarbon polymer such as typically Nafion®. The electrolytic membrane of the type has good ionic conductivity and has relatively high mechanical strength, but has some problems to be solved such as those mentioned below. Concretely, in the electrolytic membrane, water and the sulfonic acid group form cluster channels, and protons move in the cluster channels via water therein. Therefore, the ionic conductivity of the membrane significantly depends on the water content thereof that is associated with the humidity in the service environment in which the cells are driven. For poisoning reduction in the catalyst electrode with CO and for activation of the catalyst electrode therein, solid polymer fuel cells are preferably driven at a temperature falling within a range of from 100 to 150° C. However, within such a middle-temperature range, the water content of the electrolytic membrane in the cells lowers with the reduction in the ionic conductivity thereof, and it causes a problem in that the expected cell characteristics could not be obtained. In addition, the softening point of the electrolytic membrane is around 120° C. and when the cells are driven at a temperature around it, then still another problem with it is that the mechanical strength of the electrolytic membrane is unsatisfactory. On the other hand, when the electrolytic membrane of the type is used in DMFC, then it causes still other problems such as those mentioned below. Naturally, the membrane readily absorbs water and its barrier ability against the fuel methanol is not good. Therefore, methanol having been fed to the anode penetrates through the electrolytic membrane to reach the cathode. Owing to it, the cell output power lowers, and this is referred to as a methanol-crossover phenomenon, and is a serious problem. For practical use of DMFC, this is one important problem to be solved.
Given that situation, there is a growing tendency for the development of other proton-conductive materials substitutable for Nafion®, and some hopeful electrolytic materials have been proposed. For inorganic proton-conductive materials, for example, proton-conductive glass is disclosed (for example, see JP-A 2000-272932, 2000-256007, 2000-357524, 2001-93543; Journal of Physical Chemistry, B, 1999, Vol. 103, p. 9468; Physical Review, B, 1997, Vol. 55, p. 12108). This is obtained through polymerization of tetraalkoxysilane in the presence of acid in a sol-gel process, and it is known that its humidity dependency is low in a high-temperature range. However, the glass is not flexible and is extremely brittle, and large-area membranes are difficult to produce from it. Therefore, the glass is unsuitable for electrolytes for fuel cells.
For easy film formation based on the good characteristics of inorganic material, one proposal is a nanocomposite material hybridized with polymer material. For example, proposed is a method of forming a proton-exchange membrane by hybridizing a polymer compound having a sulfonic acid group in the side branches, a silicon oxide and a proton acid (for example, see JP-A10-69817, 11-203936, 2001-307752). Another proposal is an organic-inorganic nanohybrid proton-conductive material that is obtained through sol-gel reaction of a precursor, organic silicon compound in the presence of a proton acid (for example, see Japanese Patent 3,103,888, and Electrochimica Acta, 1998, Vol. 43, Nos. 10-11, p. 1301). WO03/041091 discloses a proton conducting membrane crosslinked by silicon-oxygen linkages, which is characterized by a fact that it bears a carbon-containing organic-inorganic composite structure covalently bonded to plural silicon-oxygen crosslinks and an acid-containing structure having acid groups. These organic-inorganic composite and hybrid proton-conductive materials comprise an inorganic component and an organic component, in which the inorganic component comprises silicic acid and proton acid and serves as a proton-conductive site and the organic component serves to make the materials flexible. When the inorganic component is increased so as to increase the proton conductivity of the membranes formed of the material, then the mechanical strength of the membranes lowers. On the other hand, however, when the organic component is increased so as to increase the flexibility of the membranes, then the proton conductivity of the membranes lowers. Therefore, the materials that satisfy the two characteristics are difficult to obtain. Regarding the methanol perviousness of the materials, which is an important characteristic of the materials for use in DMFC, satisfactory description is not found in the related literature.